1. Field of the Invention
The present invention relates to friction welding and, more specifically, to backing up a weld joint during friction stir welding.
2. Description of Related Art
Friction stir welding is a relatively new process using a rotating tool, which includes a threaded pin or probe attached to a concave shoulder, to join in a solid state two workpieces or to repair cracks in a single workpiece. At present, the process is applied almost exclusively in straight-line welds. For example, such a process is described in U.S. Pat. No. 5,460,317 to Thomas et al., the contents of which are incorporated herein by reference. As shown in FIG. 1A, during friction stir welding, the probe 10 of the rotating tool 12 is plunged into a workpiece or between two workpieces 14 by a friction stir welding machine (not shown) to produce the required resistance force to generate sufficient frictional heating to form a region of plasticized material. As shown in FIG. 1B, the tool 12 is typically tilted approximately 3° relative to the workpiece or workpieces 14 such that the trailing edge of the concave shoulder 16 is thrust into and consolidates the plasticized material. Upon solidification of the plasticized material, the workpieces 14 are joined along the weld joint 18. The magnitude of force exerted by the friction stir welding tool 12 must be maintained above a prescribed minimum in order to generate the required frictional heating.
To prevent deformation of a workpiece by the force exerted by the friction stir welding tool 12 and maintain dimensional tolerances, the workpiece 14 must have support 15 behind the weld joint. Additionally, because the frictional heat generated by the welding tool plasticizes the material within the weld joint, the plasticized material must be constrained to prevent the material from flowing out of the weld joint and also must be consolidated to minimize porosity and provide a weld joint having the desired surface finish. When friction stir welding relatively flat workpieces, the weld joint can be supported by a continuous planar surface, such as a steel plate, positioned underneath the workpieces to be joined.
When friction stir welding large workpieces or workpieces having curvilinear geometries, providing adequate support to the weld joint becomes problematic because the amount of support material necessary and/or the curvilinear geometry makes it more difficult and expensive to provide a continuous support surface. Such welds are often necessary when fabricating military and commercial aircraft and rocket fuel tanks. In certain instances, a built-up structure, commonly referred to as “tooling,” can be secured to the interior surfaces of the workpieces prior to friction stir welding. However, weight restrictions and/or design parameters often require a finished assembly having a smooth interior surface. As such, the tooling must be removed, for example, by machining, which is time consuming and labor intensive and increases the manufacturing cost of the finished assembly.
Another problem that has been encountered with friction stir welding occurs when joining workpieces formed of different materials having different material properties, such as solidus temperature, hardness, and/or thermal conductivity. The “solidus” temperature of a particular alloy is the temperature below which only a solid is stable. The different material properties can require that the friction stir welding tool be rotated within each workpiece at different rotational speeds and/or have different rates of tool advance through the workpieces, which can complicate the friction stir welding process and can limit the types of materials that can be joined. For example, when friction stir welding workpieces with different solidus temperatures, the friction stir welding tool will plasticize the workpiece with the lower solidus temperature first, such that the workpiece with the higher solidus temperature may not be sufficiently plasticized to be mixed with the other workpiece, as is necessary to form a strong weld joint.
Thus, there is a need for an improved friction stir welding tool for forming weld joints between large workpieces or workpieces having curvilinear geometries. The tool should be capable of effectively supporting a weld joint and constraining the plasticized material within the weld joint during friction stir welding and should be easily adaptable to varying workpiece geometries and sizes. In addition, the tool should allow for friction stir welding workpieces having different material properties.